I am a PhD candidate at Tokyo Institute of Technology. I received BS and MS from Tokyo Institute of Technology. My current research focuses on structure of glass involving molten state
and vitrification process.

Structures of glasses and melts have been investigated by many researchers because those are deeply correlated with the properties of glass. Especially, aluminosilicate system is
one of the interest materials not only in earth science but also in glass science and technology. However, those are not fully understood yet, especially from points of view of
structural alternations with composition and temperature. This study gives a focus on the MgO substitution for Na2O in aluminosilicate glass system. Magnesia has been known as
one of the key constituents to induce unique properties of glass and glass-ceramics as the materials with small thermal expansivity for structural, thermal or optical uses.
The purpose of this study is to clarify the structural role of magnesium component in aluminosilicate glass system and the correlation with its unique properties.

Glass samples with the compositions of 6.6xNa2O–(33-6.6x) MgO-12Al2O3-55SiO2 in mol%, where x = 0~5, were prepared by melt quenching method. The prepared glass sample was
re-melted in a Pt crucible at 1600 °C using a compact electric resistance furnace for Raman spectroscopy, and nanosecond Q-swithced SHG Nd:YAG laser (
λ = 532 nm) pulses were
irradiated onto the melt in the crucible. Collection of Raman signals was carried out at the temperatures from 1600 °C to room temperature at intervals of 100–200 K.

The main Raman bands appear in high wavenumber region which are attributed to the silicate units with different connectivities to other structural units. MgO substitution
is found to induce a characteristic band at around 1200 cm-1. This band intensity increases with MgO substitution (Fig. 2). From the comparison with the previous data on
cordierite (2MgO·2Al2O3·5SiO2) crystal[1], in which quantum-mechanical calculations of crystal model has been made to identify the experimental Raman bands, the observed
high-frequency band is ascribed to the vibration of the dimer of SiO4 tetrahedra connected with three AlO4 tetrahedra. This reveals that Mg2+ ions strongly interact with AlO4
tetrahedra to form unique Al-rich cluster similar to that found in cordierite crystal. Since this structural feature is observed in Raman spectra of glass melts at high
temperature, MgO plays an important role to affect glass network in wide temperature range from melting temperature to room temperature.